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Young’s Double Slit Experiment
Apparatus
optics bench laser slit film screen white paper and tape pencil metric ruler Ocean Optics spectrometer and fiber optics cable
Goal
In this experiment, you will set up a Double Slit apparatus and will measure the distance between bright fringes of the interference pattern. You will repeat the experiment for different pairs of slits to see how the interference pattern depends on slit separation. You will use a curve fit to measure the wavelength of the light.
Introduction
As a plane wave passes through a slit, it emerges as circular waves as if from a point source of light. If light from a laser (which is a plane wave) passes through two slits, then each slit acts as a point source of light. Because the incident wave is a plane wave and is monochromatic (i.e. one color, one frequency), then the waves from each slit will be in phase (meaning a crest will emanate from each slit at the same time) and will have the same frequency and amplitude. The waves from each source will interfere. At some points in space, total constructive interference occurs and a detector at this location will measure a wave with twice the amplitude, 2 A, as the wave from each source. At other points in space total destructive interference occurs and a detector at this location will measure a wave with zero amplitude (meaning no wave at all). At the majority of points in space, interference results in a wave that has an amplitude between 0 and 2 A–meaning that it is not total constructive or destructive interference. You an identify such points in Figure 1.
Figure 1: Constructive and destructive interference due to two sources.
At some points in space around the two sources, total constructive interference will occur; at these points the path difference (∆L) from the two sources is:
∆L = |L 1 − L 2 | = mλ m = 0, 1 , 2 ,... total constructive interference
where L 1 and L 2 are the distances from the two sources to a point in space. For all points where m = 0, then L 1 = L 2 and total constructive interference occurs. One such location is shown in Figure 2.
Figure 2: Total constructive interference where m = 0.
You can identify a line of points where m = 0. Sketch a line on Figure 2 that connects all points where m = 0 and total constructive interference occurs. For all points where m = 1, then |L 1 − L 2 | = 1λ and total constructive interference occurs. Two different locations where m = 1 are shown in Figure 3.
Figure 3: Total constructive interference where m = 1.
You can identify two separate lines of points where m = 1. Sketch both lines on Figure 3 that connects all points where m = 1 and total constructive interference occurs. Note that the points around the lines of total constructive interference are also bright and are a result of constructive interference, but are not as bright as the points where total constructive interference occurs. These regions are called “fringes” and it is in the center of the fringes where total constructive interference occurs. In the example above, there is a total of 5 fringes shown. As you can imagine, the pattern of points where total constructive interference occurs is related to the wavelength of the light and the distance between the sources. To explore this, you will first use a simulation.
As the distance between two sources increases, does the number of fringes increase, decrease, or remain the same?
As the distance between two sources increases, do the fringes get further apart or do they get closer together?
Experiment 2 – Measuring fringe spacing for double-slit interference
If you define a y-axis at the top of the animation window and mark the center of each fringe, then the spacing between fringes is ∆y, as shown in Figure 5. It is this fringe spacing that you will measure in this experiment.
Figure 5: Definition of fringe spacing.
When you shine a laser onto 2 slits, you will get an interference pattern for the reasons described above. You will see a pattern like the one shown in Figure 6. The graph is the intensity of light I as a function of position y on a screen. y = 0 is defined as the center of the interference pattern, along a perpendicular bisector of the two slits. Use the graph to answer
Figure 6: An interference pattern due to laser light incident on 2-slits.
the following questions.
Note that the center of the left fringe is at y = −10 mm and the center of the right fringe is at y = 10 mm.
What is the distance from the center of the far left fringe to the center of the far right fringe?
How many total fringes are on the screen? (Include the far left and far right fringes.)
What is the slit spacing ∆y between fringes? The fringe spacing ∆y depends on the wavelength of the light and the distance between the slits (d). A picture of the apparatus is shown in Figure 7.
slit spacing, d (mm)
Number of fringes (N)
distance be- tween N fringes (cm)
fringe spacing, ∆y (cm) L (m) λ (nm)
Analysis
- What is the average wavelength of the laser as determined by your measurements using Young’s Double Slit Experiment?
- What is the standard deviation of the mean wavelength?
Using a spectrometer to measure wavelength
A spectrometer is a device that uses a diffraction grating to split light into its component wavelengths. It measures the light intensity at different wavelengths. A graph of intensity vs. wavelength is used to identify different wavelengths (i.e. colors) in the spectrum. Note that it only looks at the first order (m = 1) spectrum.
- An Ocean Optics spectrometer has been set up for you at your computer. It is plugged into the computer via the USB port. It has a fiber optic cable attached to it.
- View Logger Pro. It may already be set up and configured.
- By default, Logger Pro plots absorbance vs. wavelength. But we wish to measure intensity. Logger Pro may already be set up to plot intensity vs. wavelength. If it is not, then go to Experiment→Change Units→Spectrometer: 1→Intensity to change what is being measured by the spectrometer.
- Point the laser into the end of the fiber optic cable. Use the clothespin to press the button so that the laser remains on.
- Click the Collect button. After you get a spectrum, click the Stop button. If the graph you see has a peak that goes off the graph (as shown below), that means we’ve saturated the spectrometer, and we need a way to make the laser less bright.
To do this, we can slightly tilt the laser so that less light enters the spectrometer. Do this and take another measurement. Continue trying until you get a peak that is not saturated.
- Sketch and label the graph you see (when the spectrometer is not saturated).
- You can measure the wavelength of any point on the spectrum using the Examine tool. Go to Analyze→Examine. Hover over any point on the graph and the wavelength value will be displayed. Once you hover over a selected peak, you should view the data table to confirm the wavelengths of each peak. You can also highlight the peak and click the ‘Statistics’ icon in the toolbar to obtain the wavelength of the maximum intensities.
- Record the wavelength of the maximum intensity on the data sheet. The fact that the spectrum of a laser gives a single, narrow peak means that the light from the laser is composed of a single color. Thus it is monochromatic.
Analysis
- What is the wavelength of the laser according to your measurement with the spectrometer?
- Does your measurement using the double-slit apparatus agree with your measurement using the spec- trometer? In other words, is the measurement with the spectrometer within the uncertainty of your measurement with the double-slit apparatus?
